The present invention relates to a ramark beacon apparatus for indicating direction on a marine radar screen. A ramark beacon apparatus is fixed on land, and transmits a pulse signal, which is received by a marine radar, and then, said pulse signal received by the marine radar indicates a direction marker on a radar screen.
Conventionally, a transponder, or a racon has been utilized for indicating a marker on a radar screen for indicating the fixed position on land. However, said racon has the disadvantage that it is sometimes difficult to recognize the position of the racon station since the marker by a racon is a short line on a radar screen, and that short line is sometimes difficult to recognize on a screen. Therefore, a racon is only used for locating a ship within a narrow area.
On the other hand, a ramark beacon apparatus which is fixed on land provides a long marker line extending the whole radius on a radar screen, and therefore, a ramark beacon is used for an initial location of a ship at a long distance.
FIG. 1 is a block diagram of a prior ramark beacon apparatus, which transmits a pulse signal. In the figure, the reference numeral 1 is an antenna, 2 is a transmitter, 3 is a sweep signal generator, 4 is a code generator which modulates the beacon signal with the predetermined code. That code identifies, for instance, the particular ramark beacon station. The sweep signal generator 3 is used for sweeping the transmission frequency of the ramark beacon apparatus so that the ramark beacon signal may be received by any marine radar which uses the radar frequency, for instance, in the 9400 MHz band.
The prior ramark beacon apparatus of FIG. 1 transmits a beacon signal continuously, and the code transmitted by the ramark beacon is of course not synchronized with a radar pulse on a ship. Therefore, the ramark beacon code indicated on the PPI screen (Plan Position Indication) on a ship is indicated as shown in FIG. 2, in which the combination of short dots and long dots is not synchronized with the scanning of a radar screen, and therefore, it is almost impossible to understand the code although the ramark beacon signal is modulated with the particular code. Further, the asynchronized indication of FIG. 2 has the disadvantage that the intensity of a line on a screen is weak, since short dots and long dots are distributed at random. Accordingly, the indication of a ramark beacon on a radar screen looks like a vague line, from which it is impossible to identify a code of the beacon station.